Lithium ion secondary batteries are widely used for portable electronic instruments such as cell phones or notebook-size personal computers. As a cathode active material for a lithium ion secondary battery, a cathode active material comprising a composite oxide of Li with a transition metal element (such as LiCoO2, LiNiO2, LiNi0.8Co0.2O2 or LiMn2O4) is known.
As a process for producing a cathode active material, for example, a process is known such that a sulfate of a transitional metal element such as a sulfate of Mn and a carbonate such as sodium carbonate are mixed in the form of aqueous solutions, to obtain a carbonate compound containing the transitional metal element, and then the carbonate compound and lithium carbonate are mixed, followed by firing (Non-Patent Document 1).
However, in the production of a cathode active material which employs the process as disclosed in Non-Patent Document 1, particularly in a case where the proportion of Mn is lowered in order to increase discharge capacity, the particle size, the particle shape, etc. of the cathode active material tend to be ununiform, and voids are easily formed in the cathode active material. Such a cathode active material quickly deteriorates, and it is difficult to obtain a lithium ion secondary battery which is excellent in characteristics that the discharge capacity and the average discharge voltage will not substantially decrease after repeating the charge and discharge cycle (hereinafter referred to also as “cycle characteristics”).